This invention relates to an apparatus for timing pouring of molten metal into a die of a casting machine for die-casting small articles, such as dental articles and personal ornaments.
U.S. patent application Ser. No. 09/415,282 filed on Oct. 8, 1999, now U.S. Pat. No. 6,250,367, entitled xe2x80x9cMolten Metal Pouring Timing Determining Apparatus and Casting Machinexe2x80x9d, assigned to the assignee same as the assignee of the present application discloses an apparatus similar to the apparatus disclosed in the present application. This copending application is incorporated herein by reference.
Molten metals to be cast have their own proper timings when they should be poured into a die. If molten metal is poured in a die at a time earlier than its proper pouring timing, its viscosity is too high to spread over the entire cavity in the die, so that articles cannot be cast with precision. On the other hand, if the metal is poured later than the proper pouring timing, the casting temperature is so high that the metal may be evaporated, oxidized or degraded in composition. In addition, when the metal is poured into the die, it may stick to the die because of its high temperature. Like this, the timing of pouring molten metal into the die is critical to the quality of cast articles.
Conventionally, the time at which a molten metal should be poured into a die is determined by artisans, who monitors, by eyes, the metal being melted for minute vibrations, flow, deformation, glow, color etc. of the metal, to determine when the viscosity of the entire molten metal has decreased to a viscosity suitable for pouring the metal into the die.
The proper timing of the pouring of a metal into a die is correlated to the surface temperature of the molten metal. Therefore, it has been proposed to use an infrared radiation thermometer for measuring the surface temperature of a mass of molten metal to time the pouring of the metal. It is, however, very hard to detect an accurate surface temperature of a molten metal mass with an infrared radiation thermometer because of various reasons including the following ones. First, the amount of infrared radiation emitted differs from metal to metal. In addition, for a particular metal, the surface state of the molten metal mass changes from time to time, so that the amount of infrared radiation varies from time to time, too. Furthermore, from the time at which the metal starts melting and its viscosity starts decreasing, metal films, such as an oxide film, are formed to partly cover the surface of the molten metal mass and move on the surface, which causes the amount of emission of infrared radiation detected by the thermometer to randomly vary. Also, some metals may evaporate, and the evaporated metal gas and other gas may absorb or attenuate the emitted infrared light.
Fresh metal is not always used in casting, but metal obtained by cutting off unnecessary portions of a completed cast article may be recycled. Such recycled metal has a thick oxide film on its surface, which prevents detection of correct surface temperature of the molten metal. In addition, since an infrared radiation thermometer measures the temperature only at a small point on the surface of the molten metal mass, it is not possible to know the temperature of the molten metal as a whole. In other words, it is difficult to determine when the whole molten metal attains its proper pouring temperature, with the viscosity decreased to an appropriate value.
For the reasons as above stated, when an infrared radiation thermometer is used to determine the surface temperature of molten metal, a large error may result in measured temperature, which, in turn, may result in erroneous determination of the timing of pouring of the metal into a die. Thus, an infrared radiation thermometer is not always useable to precisely time the pouring of various metals under various melting conditions.
Another possible method to determine the optimum time for pouring may be to compare the shape of a mass of metal exhibited when it is heated and melted to flow with the shape of the mass of the metal when it is solid. However, this method is not applicable to some metals and recycled metals since they have a thick or hard oxide film on their surfaces, and, therefore, the shape or appearance changes only little even when the interior has melted and liquefied enough. This may cause the metals to be heated more than necessary, leading to defective casting.
Another problem in prior art is that when a plurality of solid lumps of metal are placed in a vessel for melting, they may melt in different times and in different ways, and, therefore, it is not possible or difficult to determine when all the metal lumps have melted into a uniform molten mass only from shape or appearance changes.
Because of the problems described above, it has been very difficult to reliably time pouring of molten metal in a casting machine under any of various melting conditions.
An object of the present invention is to provide an apparatus for timing to pour molten metal into a die.
According to one aspect of the present invention, an apparatus for timing the pouring of metal into a die of a die-casting machine is provided, which includes a melting vessel for receiving a metal material therein. Heating means heats the melting vessel by radio-frequency (RF) induction heating with a RF signal amplitude-modulated with a low frequency signal. A light receiver receives light emitted by the metal material in the melting vessel and develops a received-light-representative signal. Frequency component extracting means extracts a frequency component resulting from a sudden change in the received-light-representative signal. A pouring command signal generator generates a pouring command signal when the output signal of the frequency component extracting means exceeds a reference signal.
Radio-frequency induction heating has the following four characteristics.
(1) Immediately after entire metal is liquefied, the liquefied metal is electromagnetically stirred due to induction heating, which causes vibrations in the molten metal, which, in turn, results in a steep rising in the received-light-representative signal.
(2) A metal lump is heated from the outer portion thereof, and hot areas expand toward the center of the lump. Immediately before the electromagnetic stirring of the molten metal begins, the center portion of the lump is heated to a high temperature. At the time when the entire lump liquefies, the amount of light emitted by the liquefied increases rapidly, resulting in increase of the received-light-representative signal.
(3) At the time when the electromagnetic stirring begins, the surface state of the molten metal may suddenly changes. For example, the oxide film over the molten metal may partially broken, or a large opening may be formed in the oxide film. This causes an abrupt change in the amount of light emitted from the molten metal, resulting in increase of the received-light-representative signal.
(4) The amount of light emitted from the surface of the molten metal may suddenly change due to some other reasons, which also results in increase of the received-light-representative signal.
One or more of these four phenomena occur simultaneously around the time when the entire molten metal is liquefied. All of these phenomena appear as an abrupt change in the amount of emitted light or vibrations of the molten metal, either of which results in a rapid rising in a signal representing light received by the light receiver. This rapid rising in the received-light-representative signal is extracted with the frequency component extracting means to determine a proper time at which the molten metal should be poured into the die of the die-casting machine.
The pouring command signal generator may be arranged to generate a pouring command signal when the output signal from the frequency component extracting means continues to be above the reference signal for a predetermined time period. A change similar to a change which would appear in the received-light-representative signal when metal has been liquefied appears instantaneously when a heated lump of metal which has not yet been liquefied and, therefore, has a relatively low temperature moves in the vessel for some reason. The pouring time determined based on such change is not proper time. To avoid it, the pouring of molten metal is timed based only on a change in the received-light-representative signal caused by vibrations or abrupt increase in emitted light lasts a predetermined time.
The pouring command signal generator may include a comparator which develops an output signal only during a time period when the output signal of the frequency component extracting means is above the reference signal. When the comparator develops an output signal, the heating means operates to amplitude-modulate the RF signal with a low frequency signal, and the frequency component extracting means extracts the low frequency signal. Alternatively, the heating means may be so arranged as to amplitude-modulate the RF signal with a low frequency signal all the time throughout the operation, and increase the amplitude-modulation factor when the comparator develops an output signal. In the latter case, the modulation factor is initially small, and, therefore, the modulation is not detected by the frequency component extracting means.
With this arrangement, a rapid change in the received-light-representative signal causes the amplitude-modulation of the RF signal with a low frequency signal to be started, or the modulation factor to increase. If metal lump which has not yet been melted well moves, e.g. falls down, a rapid change cased in the received-light-representative signal by such movement is only instantaneous, and, therefore, the starting of the modulation of the RF signal with the low frequency signal, or an increase of the modulation factor, causes no pouring command signal to be generated. On the other hand, if the metal has been already well melted, the modulation with the low frequency signal or the increase of the modulation factor causes the molten metal to continuously vibrate due to the amplitude-modulation and electromagnetic stirring, which, in turn, causes the change in level of the received-light-representative signal to continue. This, in turn, causes the comparator to continuously develop the output signal, causing the pouring command signal to be developed. This arrangement can determine a proper pouring time more reliably.
The pouring command signal generator may include, in addition to the comparator which develops an output signal only during a time period in which the output signal of the frequency component extracting means is above the reference signal, a timer and a timer setting unit. In an automatic mode of operation of the molten metal pouring time determining apparatus, the timer develops the pouring command signal when the comparator continues to develop an output signal for a preset time period, and the timer setting unit sets in the timer, the time from the start of occurrence of the comparator output signal to the occurrence of the pouring command signal, measured in a manual mode of operation.
How to determine the time length for which the frequency component extracting means should continue developing its output signal, before molten metal is to be poured into a die, is highly experiential. Accordingly, in the manual operation mode, the time length is measured from the time when the output signal of the frequency component extracting means exceeds the reference signal to the time which the experienced operator judges is the time to pour molten metal into a die, causing the pouring command signal to be manually developed and the time measurement to be stopped. The measured time period is set in the timer. Thus, in the automatic mode of operation, the set time period determined by the experienced operator is used to determine the time to pour molten metal into a die.
The molten metal pouring time determining apparatus according to the present invention may include reference signal holding means. The reference signal holding means holds as the reference signal, the received-light-representative signal at the time when the rate of change of the received-light-representative signal is maximum or minimum.
Depending on the melting temperature, amount, shape and attitude in a melting pot or vessel of metal to be melted, the amount of light emitted and received from molten metal differs. Also, gas generated by the metal and stains on the melting pot can affect the amount of light received. Therefore, the level of the received-light representative signal differs, accordingly. Accordingly, the reference signal should be changed in accordance with the level of the received-light representative signal.
In general, increase in temperature of metal is relatively rapid immediately after the beginning of heating. The rate of change decreases before the start of the melting, and the increase in temperature almost stops from the beginning of the melting until the metal is completely melted. The change of the received-light representative signal exhibits a tendency corresponding to that of the temperature increase of the metal melted in the melting pot, with the level of the signal depending on the above-described various causes.
For this reason, the received-light representative signal at the time when the rate of change of the received-light representative signal is maximum (i.e. the time immediately following the start of heating) or minimum (i.e. the time when the melting begins) is held as the reference signal. With this arrangement, the precision for timing the pouring of molten metal does not decrease regardless of the above-described causes for changing the signal level because the reference signal changes, corresponding to such causes.
The amplitude modulation provided by the heating means may be stopped in response to the generation of the molten metal pouring command signal. The pouring command signal can be used as a drive signal for driving an arrangement to cause the molten metal to be poured from a melting pot into a die. An xe2x80x9cagingxe2x80x9d time may be disposed before driving the pouring arrangement so that substantially the entire molten metal can have the same temperature and viscosity. During this aging time, the modulation with a low frequency signal of the RF heating signal is interrupted so that the molten metal in the melting pot can be stabilized and stationary, whereby the molten metal can be uniformly poured into a die.
A casting machine can be provided by combining a molten metal pouring command unit with the molten metal pouring time determining apparatus, which causes molten metal to be poured into a die from a melting pot in response to the molten metal pouring command signal generated by the molten metal pouring time determining apparatus.